In a typical cellular radio communication system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennas in the cells are in turn coupled to a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) and/or a packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the PSTN or a packet-switched network (e.g., the Internet).
A subscriber (or user) in a service provider's wireless communication system accesses the system for communication services via an access terminal, such as a cellular telephone, pager, or appropriately equipped portable computer, for instance. When an access terminal is positioned in a cell, the access terminal (also referred to herein by “AT”) communicates via an RF air interface with the BTS antenna of the cell. Consequently, a communication path or “channel” is established between the AT and the transport network, via the air interface, the BTS, the BSC and the switch or gateway.
Functioning collectively to provide wireless (i.e., RF) access to services and transport in the wireless communication system, the BTS, BSC, MSC, and PDSN, comprise (possibly with additional components) what is typically referred as a Radio Access Network (RAN). In practice, the RAN may be organized in a hierarchical manner, with multiple BTSs under the control of a single BSC, multiple BSCs linked to a single MSC, and multiple MSCs in one region or metropolitan area connecting to RANs in other regions or metropolitan areas by way of gateway MSCs or other inter-regional switches.
As the demand for wireless communications has grown, the volume of call traffic in most cell sites has correspondingly increased. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennas. These sectors can be referred to as “physical sectors,” since they are physical areas of a cell site. Therefore, at any given instant, an access terminal in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS serving that physical sector.
The functional combination of a BTS of a cell or sector with a BSC is commonly referred to as a “base station,” although the actual physical configuration can range from an integrated BTS-BSC unit to a distributed deployment of multiple BTSs under a single BSC, as described above. Regardless of whether it is configured to support one cell, multiple cells, or multiple sectors, a base station is typically deployed to provide coverage over a geographical area on a scale of a few to several square miles and for tens to hundreds to several thousands (or more) of subscribers at any one time. On this scale, coverage is referred to as “macro-network coverage” and the base station is referred to as a “macro-type base station.”
More recently, a type of base-station functional unit aimed at coverage over a much smaller physical area and at concurrent support of many fewer subscribers has been introduced. Referred to generically herein as a “micro-type base station,” this device, roughly comparable in size to desktop phone, can operate to fill in gaps in macro-network coverage (e.g., in buildings), as well as provide limited and exclusive coverage to individual subscribers within residential (or other small-scale) spaces. As such, macro-network service providers have begun offering micro-type base stations as consumer devices, under the more technical moniker of “femtocell.” In addition to “femtocell,” other terms for a micro-type base station used interchangeably herein include “femto base station,” “femto BTS,” “picocell,” “pico BTS,” “microcell,” “micro BTS,” and “Low-Cost Internet Base Station” (“LCIB”). The prefixes “femto,” “pico,” and “micro” are also used herein to refer correspondingly to respective coverage areas. Note that “low-cost” is not intended to be limiting; that is, devices of any cost may be categorized as LCIBs, although it may be expected that most LCIBs will typically be less expensive on average than most macro-network base stations.
A typical femtocell may be considered to be essentially a low-power, low-capacity version of a macro-type base station, providing the same RF interface for wireless access, only for a much smaller physical coverage area. However, instead of connecting directly to an MSC, PDSN, other network switch, a femtocell communicates with the service provider's network via one or another form of broadband connection associated with or available to the consumer-owner (or renter) of the femtocell. With respect to a subscriber's wireless access, the small coverage area of a femtocell is viewed by the wireless communication system in the same manner as any other macro coverage area (e.g., cell or sector). In particular, a subscriber may move between neighboring coverage areas of macro-type base stations and femtocells, and even between neighboring coverage areas of different femtocells, in the same way the subscriber moves between neighboring macro coverage areas.
More specifically, as a subscriber at an access terminal moves between wireless coverage areas of a wireless communication system, such as between cells, sectors, or femto coverage areas, or when network conditions change or for other reasons, the AT may “hand off” from operating in one coverage area to operating in another coverage area. In a usual case, this handoff process is triggered by the access terminal monitoring the signal strength of various nearby available coverage areas, and the access terminal or the BSC (or other controlling network entity) determining when one or more threshold criteria are met. For instance, the AT may continuously monitor signal strength from various available sectors and notify the BSC when a given sector has a signal strength that is sufficiently higher than the sector in which the AT is currently operating. The BSC may then direct the AT to hand off to that other sector. By convention, an AT is said to handoff from a “source” cell or sector (or base station) to a “target” cell or sector (or base station).
In some wireless communication systems or markets, a wireless service provider may implement more than one type of air interface protocol within a single system. For example, a carrier may support one or another version of CDMA, such as EIA/TIA/IS-2000 Rel. 0, A (hereafter “IS-2000”) for both circuit-cellular voice and data traffic, as well as a more exclusively packet-data-oriented protocol such as EIA/TIA/IS-856 Rel. 0, A, or other version thereof (hereafter “IS-856”). In such a “hybrid system,” an access terminal might not only hand off between coverage areas under a common air interface protocol (e.g., between IS-2000 sectors) but may also hand off between the different air interface protocols, such as between IS-2000 and IS-856. An access terminal capable of communicating on multiple air interface protocols of a hybrid system is referred to as a “hybrid access terminal.” Handoff between different air interface protocols (or, more generally, between different access technologies) is known as “vertical” handoff.